1. Field of the Invention
The present invention relates to a light weight cement composition made from sea water which is used in sealing or cementing subterranean zones penetrated by a well bore, such as cementing the annular space between an oil and/or gas well casing and the surrounding formation.
2. Description of the Prior Art
In the production of hydrocarbons from the subterranean formation, the subterranean formation is typically cemented or sealed by pumping an aqueous hydraulic cement slurry into the annulus between the pipe and the formation. In the oft practiced placement of cement in the annular space between the casing of an oil well and the surrounding subterranean environment, the cement slurry is commonly pumped down inside the casing and back up the annular space surrounding the casing. Occasionally, the cement is introduced directly into the annular space at the outer side of the casing. Where the cement has been pumped down the casing initially, any cement slurry which remains in the casing is displaced into the annulus by a suitable fluid or fluids.
During cementing operations where fresh water is either unavailable or expensive, sea water is often utilized to make the cement slurry used in the cementing operation.
When sea water is used in the preparation of a cement slurry, certain detrimental effects can occur. For example, cement slurries containing sea water generally require large concentrations of retarders, dispersants, and fluid loss additives to achieve the same performance observed in corresponding fresh water slurries. Furthermore, when sea water is utilized in the preparation of a light weight cement slurry, soluble silicates can be used to suspend the solids of the cement in the presence of the excess sea water. In light weight sea water slurries, a large concentration of the silicate is required. This increase in the required soluble silicate level shortens the pumping time of the cement slurry even in the presence of a cement set retarder and can prevent the slurry from remaining fluid for the time necessary to place it in the well bore annulus. If the amount of silicate is reduced to the level which will allow it to remain fluid for extended periods of time at elevated temperatures, the sea water will cause the slurry to settle after it is placed in the well bore annulus. This results in intervals of water in the annular space between the casing and the formation rather than a competent cement slurry of uniform density. Thus, in conventional cementing applications, a compromise between the concentrations of added soluble silicate and cement set retarder is required in order to minimize settling while still yielding a slurry with adequate pumping time. Without corrective measures, this compromise is difficult, if not impossible to achieve.
In general for all cement slurries, it is believed that the difficulty of using sea water to form a cement slurry is due to magnesium ions which are present in the sea water, but it is to be expressly understood that the invention herein shall not be unduly limited by this proposed theory whether or not it be correct. It has been found that sea water can contain from about 1000 to about 1700 ppm of magnesium compounds (calculated as elemental magnesium). When water containing magnesium ions is utilized to form a cement slurry, it is believed that the magnesium ions are precipitated through a reaction with hydroxide compounds released in the initial hydration of the cement components. The decrease in hydroxide concentration may alter the solubility of the various components dissolved in the aqueous phase of the cement slurry, especially in the immediate vicinity of the hydrating cement particle and this, in turn, may alter the type, rate, and extent of the initial hydration reactions of the cement particles in such a manner that substantial gellation or premature settling of the cement slurry occurs. An alternative or additional theory for the detrimental effect of magnesium ions when present in cement slurries, is that precipitated magnesium hydroxide initially possesses an extremely large active surface area and that various components of the cement slurry, are absorbed onto the surface of the precipitated magnesium hydroxide and are effectively removed from solution.
These theories are put forth in order to explain the influence of sea water on the response of cement slurries in general. For light weight cement slurries prepared with soluble silicates and sea water, an additional phenomena is involved. The field preparation of such slurries normally involves the addition of the soluble silicate to the sea water prior to the addition of the hydraulic cement. Since the sea water contains both magnesium ions and calcium ions and since the pH of the sea water is low relative to the soluble silicate, a sizable portion of the silicate, which is added to the sea water, is removed from solution through the precipitation as magnesium or calcium silicates and/or by the insolubilization of the silicate due to the relatively low pH of the sea water. These reactions require an increase in the level of soluble silicate because of the silicate which was removed from solution by the sea water. Unfortunately, the precipitated and insolubilized silicates are still active with regard to their influence on the response of the cement slurry and require the addition of larger amounts of cement set retarders in order to achieve the pumping times of corresponding fresh water slurries. Unfortunately, the increases in cement retarder concentration and type of retarder required in such a situation tends to negate the suspending properties of the soluble silicates. Thus, it is often difficult, if not impossible, to arrive at concentrations of cement set retarder and soluble silicates which effectively compromise the pumping time and particle settlement requirements for many applications.
The present invention is a light weight cement composition containing silicate and made from sea water, a method of preparing the cement composition, and a method of sealing or cementing a subterranean formation using the cement composition which overcomes or at least mitigates the above-described problems.